Process of making acousto-optic devices

ABSTRACT

Acousto-optic devices comprising a transducer bonded to an acousto-optic medium having improved bonds are made by a process whereby the surfaces to be joined are first roughened, a primer metal layer is coated onto the acousto-optic medium, which primer metal is the metal which is chemically bonded to oxygen in the acousto-optic medium, the primer layer is cooled, a layer of a conductive metal and a layer of indium are deposited onto both surfaces to be bonded and the indium layers pressed together in a vacuum.

United States Patent. 11 1 Alphonse et al.

[ Mar. 26, 1 974 PROCESS OF MAKING ACOUSTO-OPTlC DEVICES Inventors:Gerard Argant Alphonse,

Hightstown; George Edward Bodeep, Wall Township,'both of NJ.

Assignee: RCA Corporation, New York, NY.

Filed: Oct. 10, 1972 Appl. No.: 295,975

1m.c1 ..B23k 21/00 Field of Search 29/4729, 473.1, 470.1, 29/4727, 594

References Cited 5 UNITED STATES PATENTS 3/1954 Brew 29/470.l UX

5/1955 Ziegler v 29/4727 X 10/1960 Kreuchen....

5/1964 Allen 29/4727 X 4/1966 Allen 29/470.l X

TRANSDUCER 7/1971 Chase et al. 29 504 x OTHER PUBLICATIONS Larson, "1,John D. et al Ultrasonically Welded Piezoelectric Transducers, IEEETransactions on Sonics and Ultrasonics, July, 1971. pp. 142-152.

Primary Examiner-J. Spencer Overholser Assistant Examiner-Ronald J.Shore Attorney, Agent, or Firm-Glenn l-l. Bruestle; Birgit E. Morris [57] ABSTRACT Acousto-optic devices comprising a transducer bonded to anacousto-optic medium having improved bonds are made by a process wherebythe surfaces to be joined are first roughened, a primer metal layer iscoated onto the acousto-optic medium, which primer 5 Claims, 1 DrawingFigure PATENTEDMARZB m4 TRANSDUCER ACOUSTO-OPTIC MEDIUM PROCESS OFMAKING ACOUSTO-OPTIC DEVICES This invention relates to an improvedprocess for bonding a transducer to an acousto-optic medium and todevices made by such process.

BACKGROUND OF THE INVENTION Since the amount of sound which enters theacousto-.

optic medium depends in part on the quality of the bond between thetransducer and the acousto-optic medium, the strength of the bond isimportant. The bonding materials used heretofore have included lowmelting inorganic solders, varnish, epoxy resins, silicone oils andgreases. These bonding materials are mismatched acoustically to theacousto-optic medium, and, in order to prevent large losses, the bondinglayer must be kept thin, e.g., a few thousand Angstroms or less. Themechanical strength of such thin bonding materials has been low,resulting in bond failure during lapping or handling.

Sittig and Cook, Proceedings IEEE, 56 p. 1375 et seq. (1968) disclosebonding of a sodium potassium niobate transducer to an acousto-opticmedium by polishing and cleaning the surfaces to be joined, coating witha thin chromium layer as a metal primer layer, next coating with a thingold layer and finally with a thin indium layer and pressing underseveral thousand psi at room temperature. However, this bond hasinsufficient mechanical strength to withstand the strains of lapping toreduce the size of the transducer. Heating at temperatures of about200300C. can improve the strength of the bond, but has an adverse effecton the properties of the acousto-optic medium. Thus an improved bondbetween a transducer and an acousto-optic medium would improve theperformance of these devices.

SUMMARY OF THE INVENTION surfaces to be joined are, first, roughenedslightly. The

primer metal is the metal chemically bonded to oxygen in theacousto-optic medium and the primer' metal layer is cooled prior tocoating with a conductive layer.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a sectional view of anacoustic-optic device shown prior to pressing the surfaces to be joinedtogether.

DETAILED DESCRIPTION OF THE INVENTION The transducer and acousto-opticmedium described herein are made of conventional materials andconfiguration. Transducers can be made oflithium niobate, sodiumpotassium niobate, lithium gallate, quartz, leadtitaniumdioxide-zirconia ceramic and the like. Acousto-optic media includeferroelectric materials such as lead molybdate, lead tungstate, bariumsodium molybdate, and other high refractive index materials such astelluride glass and the like.

In preparing the acousto-optic devices of the invention, the surface ofthe transducer and the acoustooptic medium to be bonded together arefirst rough: ened slightly. This improves the bond and preventsdelamination during lapping. The surfaces are abraded using aconventional abrasive, such as finely divided alumina, diamond dust andthe like. The roughened surface must be smooth to sound waves, but roughenough so that they will deflect light; undulations on the surface aboutone micron deep are satisfactory.

The surface of the acousto-optic medium is next coated with a thin layerof a primer metal, which is the metal chemically bonded to oxygen in theacoustooptic medium. For example, when a lead molybdate acousto-opticmedium is employed, the primer metal is molybdenum; when a leadtungstate acousto-optic medium is employed, the primer metal istungsten, and so forth. The primer metal coating is suitably from200-500 A thick. Preferably, the primer metal coating can also beapplied to the roughened surface of the transducer. The primer metalcoatings can be conveniently applied by vapor deposition in a vacuumchamber.

The primer metal layer is next cooled to room temperature in an inertatmosphere. This step is important in order to form a bond of acceptablemechanical strength. Conveniently, the transducer is allowed to cool inthe vacuum chamber employed for vapor deposition of the primermetal.

A thin layer of a conductive metal, for example, gold or silver, is nextapplied both to the transducer and to the acousto-optic medium over theprimer. metal. This layer is conveniently applied by vapor deposition inconventional manner until a layer of about 1,000-5 ,000 A in thicknessis obtained.

A final layer of a room temperature-weldable metal, such as indium, isdeposited on the conductive metal. This layer is also suitably fromabout l,000-5,000 A thick.

The coated surfaces of the transducer and the acousto-optic medium arepressed together under pressure of about 2,000l0,000 psi, preferably4,0005,000 psi, in a vacuum at ambient temperatures whereby they arebonded together.

Referring now to the FIGURE, the transducer 10 has a roughened surface11, a layer of primer metal 12, a layer of gold 13 and a layer of indium14.

The acousto-optic medium 15 has a roughened surface 16, a layer ofprimer metal 17, a layer of gold 18, and a layer of indium 19. Duringformation of the device, the indium surfaces 20 and-21 are pressedtogether until a single indium layer is formed.

If desired, the transducer can be reduced in size or lapped to athickness of about 15 microns without delaminating. Lapping can beaccomplished by abrading or grinding the transducer with a suitableabrasive until the desired thickness is obtained.

A device suitable for an acousto-optic deflector was prepared asfollows: a lead molybdate block and a lithium niobate transducer platewere abraded with dia-' mond dust having an average particle size ofabout 6 microns to roughen the surfaces to be bonded. They were placedin a vacuum chamber which contained three boats filled with molybdenum,gold and indium, respectively, so that the roughened surfaces werefacing the boats. The chamber was evacuated and heat applied to themolybdenum boat until a layer of molybdenum about 200 A thick had beendeposited on the roughened surfaces of the block and transducer. Afterthe molybdenum layer had cooled to room temperature, heat was applied tothe gold boat until a layer of gold about 1,000 A thick had beendeposited over the molybdenum layers. Heating of the gold boat wasdiscontinued and a final layer of indium about 1,000 A thick wasdeposited over the gold layers in similar manner. While still in thevacuum chamber, the surfaces of the acousto-optic medium and thetransducer coated as above were pressed together under about 4,000 psifor one-half hour. The resultant device was firmly bonded.

The acousto-optic device was completed by lapping the transducer to athickness of about 15 microns. N delamination was noted.

The strength of the bond between the molybdenum layer and theacousto-optic medium can be demonstrated by the Scotch tape test wherebya piece of adhesive cellophane tape was pressed firmly onto themolybdenum and the tape pulled off quickly. No delamination or flakingof the molybdenum layer was noted.

As a comparison, a 200 A thick layer of chromium deposited on leadmolybdate delaminated during Scotch tape testing.

We claim:

1. A method of preparing an acousto-optic device comprising aferroelectric transducer bonded to an acousto-optic medium whichcomprises 1. roughening the surfaces of said transducer and saidacousto-optic medium to be bonded such that the surfaces diffract lightwaves but are smooth to sound waves, i

2. depositing a primer metal layer onto the surface of saidacousto-optic medium to be bonded, which primer metal is the metalchemically bonded to oxygen in said acousto-optic medium,

3. cooling said primer metal layer,

4. depositing a conductive metal layer onto said primer layer and ontosaid roughened transducer surface to be bonded,

5. depositing an indium layer on each of said conductive layers, and I6. bonding said acousto-optic medium and said transducer by pressingsaid indium coated surfaces together in a vacuum at ambient'temperaturewith from about 2,000l0,000 psi.

2. A method according to claim 1 wherein said acousto-optic medium islead molybdate and said primer metal is molybdenum.

3. A method according to claim 2 wherein said molybdenum layer is fromabout 200-500 A thick, said conductive metal layers are from about1,000- 6,000 A thick and said indium layers are from about 1,000-5,000 Athick.

4. A method according to claim 3 wherein said conductive layer is gold.

5. A method according to claim 1 wherein said bonded transducer islapped down to about 15 microns or less.

2. depositing a primer metal layer onto the surface of saidacousto-optic medium to be bonded, which primer metal is the metalchemically bonded to oxygen in said acousto-optic medium,
 2. A methodaccording to claim 1 wherein said acousto-optic medium is lead molybdateand said primer metal is molybdenum.
 3. A method according to claim 2wherein said molybdenum layer is from about 200-500 A thick, saidconductive metal layers are from about 1,000-5,000 A thick and saidindium layers are from about 1,000-5,000 A thick.
 3. cooling said primermetal layer,
 4. depositing a conductive metal layer onto said primerlayer and onto said roughened transducer surface to be bonded,
 4. Amethod according to claim 3 wherein said conductive layer is gold.
 5. Amethod according to claim 1 wherein said bonded transducer is lappeddown to about 15 microns or less.
 5. depositing an indium layer on eachof said conductive layers, and
 6. bonding said acousto-optic medium andsaid transducer by pressing said indium coated surfaces together in avacuum at ambient temperature with from about 2,000-10,000 psi.